1. Field of the Invention
This invention relates generally to airborne threat monitoring systems, and more specifically to systems employing bi-static continuous wave radar to enable an aircraft to sense incoming ballistic threats and initiate responsive countermeasures.
2. Related Art
The evolving face of global terrorism has crystallized the threat that inexpensive and readily available shoulder-fired missiles can be directed against civilian aircraft. The FBI estimates that since 1989, more than 50,000 shoulder-fired missiles, or MANPADS (man-portable air-defense systems), have been sold to third-world countries, and that from 1978–1998, 29 civilian planes have been shot down by shoulder-fired missiles. More recently, the war in Iraq has provided a proving ground for this tactic, where terrorist militia have struck at least twelve aircraft with shoulder-fired missiles in a three-month time period from October 2003 to January 2004. These targets have included assault helicopters such as the AH-64 Apache, UH-60 Black Hawk, and OH-58 Kiowa, and also passenger-capable aircraft such as the CH-47 Chinook helicopter, C-5 and C-17 transport planes, and the DHL cargo plane.
Shoulder-fired weapons such as the Soviet designed rocket-propelled grenade (RPG) and SA series surface-to-air missiles have been used effectively in many of these assaults. The Soviet RPG-7, a relatively inexpensive device, is the most widely distributed shoulder-fired missile system in the world. It is a lightweight, requires little training, and can propel an 85-mm warhead to a range of about 300 m. The Soviet-made SA-7 surface-to-air missile can reach altitudes of up to 12,000 feet and can be launched more than two miles away from a target. The SA-7 incorporates a heat-detection targeting system, and carries a 1.6 inch warhead. The SA-7, or variants thereof, are manufactured in Pakistan, North Korea, and Cuba, from which sales of the weapon have been traced to at least 17 terrorist organizations and 56 countries. The Soviet-made SA-16, a more advanced surface-to-air missile, incorporates an infrared guidance system. It has a 72 mm warhead, a range of about 5000 m, and a maximum altitude of about 3500 m. At least 34 countries are believed to be in possession of the SA-16. The RPG and SA series weapons relatively light weight, about five to six feet long, and can be smuggled inside a large duffle bag. In the hands of terrorists, shoulder-fired munitions such as these represent a serious security threat to civilian aircraft.
While a number of sophisticated missile warning and countermeasure systems are available for use on military aircraft to detect and defend against such threats, a need exists for an effective solution in civilian applications. A number of approaches that address this problem have been explored, but none have been successfully developed for widespread use. U.S. Pat. No. 5,600,434 to Warm et al., “Apparatus for Defending Against an Attacking Missile,” describes a system that applies a pulsed laser source, directed by optical tracking, onto an incoming missile. The laser energy is emitted toward the attacking missile in order to disturb the missile's optronic detector function in hopes of causing the missile to lose track. U.S. Pat. No. 5,361,069 to Klimak et al., “Airborne Radar Warning Receiver,” describes an approach to rapidly indicate when an aircraft has entered a ground based track-while-scan radar sector and to indicate the angular position of the aircraft within this sector. U.S. Pat. No. 6,369,885 to Brown et al., “Closed-loop Infrared Countermeasure System Using High Frame Rate Infrared Receiver,” describes a system which provides simultaneous tracking and identification/classification functions with an infrared receiver having a focal plane array. The invention provides variable imaging rates to detect, jam and divert an incoming infrared missile. U.S. Pat. No. 6,137,436 to Koch, entitled “Alarm Sensor, in Particular for a Target Tracking Apparatus,” describes a system which radiates a pseudo-noise-modulated spread spectrum into displaced spatial sectors, and applies cross correlation of the received reflected energy to provide a spherical monitoring effect to warn against an attacking guided missile. U.S. Pat. No. 5,560,567 to Hallmark, “Passive Missile Tracking and Guidance System,” describes a scanning or staring infrared detection system in which the target and missile are optically sensed and the measured displacement is utilized in conjunction with calculated nominal trajectory data to generate guidance control signals. U.S. Pat. No. 5,424,744 to Westphal for a “Sensor Arrangement for Sensing a Threat” describes a passive warning system that exploits blocking effects as a threat intervenes in the line of sight between one of a plurality of signal sources (specifically sources onboard a satellite) and a receiver. The approach can distinguish threat direction based on the signal source that is obscured.
One major obstacle to development of any of the aforementioned missile warning and countermeasure systems for non-military applications is the cost of upgrading. Owners and operators of civilian and small commercial aircraft enterprises are not able to afford the costs of such sophisticated protection systems. Compatibility problems must also be overcome, due to wide variations in the design of civilian aircraft. In other cases, spatial considerations may discourage installation of a warning and countermeasure system on an aircraft where the availability of space for retrofit systems is already at a premium. A practical solution to counter ballistic threats to civilian aircraft has yet to be fully developed.